1. Field of the Invention
This invention relates to apparatus and methods for fabricating semiconductor integrated circuits and, more particularly, to apparatus and methods of preventing accumulation of developer solution on the backside of a semiconductor wafer.
2. Prior Art
In the fabrication of semiconductor integrated circuits, a layer of photoresist material is formed on the front surface of a semiconductor wafer. Using photolithographic techniques, various portions of the layer of photoresist material are selectively exposed to light using photo masks to thereby define various features on the wafer. The exposed photoresist material on the wafer is subsequently developed at a developing station using a developer solution, which removes exposed photoresist material, leaving a pattern in the remaining photoresist material for subsequently processing of the exposed areas of the wafer. At a photoresist developing station the wafer is held on a rotatable vacuum chuck, which contacts the backside of a wafer. Initially, the developer solution is dispensed onto the front surface of a slowly rotating, or spinning, wafer for initial wetting and even distribution of the developer solution. The spinning is stopped, and more developer solution is puddled onto the wafer surface and held for a period of time to complete the development step. The spent developer solution is subsequently removed from the wafer by rinsing the frontside and backside of the wafer with water. The water is removed by spinning the wafer at high speed and during a subsequent baking step for hardening the photoresist material.
During the developing step, a wafer is held in the vacuum chuck by vacuum pressure being applied to the backside of the wafer. Because of the vacuum pressure, developer solution tends to be drawn to the backside of the area adjacent to the vacuum chuck, where the developer solution is trapped between the surface of the chuck and the backside of the wafer. This trapped developer solution is not removed by the water rinse. Wafers which have developer solution on their backsides are called wetbacks. The problem with wetbacks is that the developer solution is a base-type material which will not dry, even when baked during the photoresist hardening step. The developer material can contaminate processing equipment, such as furnaces, in subsequent processing steps. The presence of residual developer material on the backside of a wafer also prevents a wafer from easily sliding in wafer-handling equipment. The wetback problem is a serious one and has been solved with varying degrees of effectiveness.
One prior-art solution to the wetback problem is the so-called air ring system. This system provides a ring having a surface which is held in very close proximity to the backside surface of a wafer. Pressurized air flows from inside the ring through a small gap between the surface of the ring and the surface of the backside of the wafer. As the air flows through the small gap, the air flow increases in velocity. In theory, this high-velocity air prevents the developer solution from entering the vacuum-pressure region between the chuck and the backside of a wafer, to prevent wetbacks. However, I have found that wetbacks still occur when this air ring technique is used. There are two problems with the air ring technique which explains how wetbacks still occur with the air ring technique.
The first problem concerns the difficulty maintaining the ring in close proximity to the backside of a wafer to maintain an optimum spacing in the air gap. A clearance of 0.005 inches is specified for the air gap. In a production environment, this clearance has been difficult to maintain because of mechanical vibrations associated with the developer process equipment.
The second problem concerns the difficulty with keeping the wafer and the ring coplanar. For optimum operation, the gap spacing must be maintained uniformly around the entire circumference of the ring. To maintain a uniform gap spacing requires an optimum chuck and spindle assembly. In addition, the ring must be mounted perfectly orthogonal to the spindle.
Because of the vibrations, the ring does not maintain proper clearance with the backside of a wafer. This results in frequent wetbacks or, worse, scratches on the back of a wafer from contact with the air ring. In practice, setup and maintenance of the air ring equipment for proper operation is difficult.
In the future, as lithography geometries in the integrated-circuit fabrication art continue to shrink, two additional problems will become more important. One of these problems has to do with developer solution being thrown from a wafer during rinse and dry cycles to form a mist. To prevent this mist from settling on the frontside of a wafer, vapors from the equipment are exhausted from the bottom of the equipment, away from the frontside of a wafer being processed. An air ring system also provides a flow of air into this exhaust stream, which decreases the efficiency of the exhaust system. Another problem has to do with the observation that contamination particles are more likely to be found in turbulent airstreams rather than in still air or in smoothly flowing airstreams. The air ring technique to reduce wetbacks uses a turbulent air flow and is therefore more likely to contain contaminants.
Consequently, the need exists for an effective technique of preventing wetbacks during the photoresist-development step of semiconductor wafer processing. That is, the needs exist for preventing develope solution from being available to be drawn by vacuum pressure to the area between the backside of a wafer being developed and the vacuum chuck on which the wafer is mounted.